This invention relates to spark plugs and more particularly to an improved spark plug for use in an internal combustion engine having a capacitive discharge ignition system.
High voltage ignition circuits for spark-ignited internal combustion engines commonly produce spurious radiation in the radio frequency spectrum. It is desirable to reduce or eliminate such radio frequency radiation since it may interfere with communication systems, navigation systems and other systems using the same radio frequency spectrum. Various techniques have been used in the past for suppressing radio frequency radiation from high voltage ignition circuits. These techniques generally consist either of placing an ignition noise suppression element in either the high voltage ignition cables or in the center electrode of each spark plug or of distributing an ignition noise suppressor in the high voltage ignition cables. For ignition systems of the Kettering type which include a set of cam driven breaker points which periodically interrupt current flow to the primary winding of an ignition coil, the ignition noise suppressor generally comprises a resistor either of the carbon type or of the wire wound type. Resistors of up to 10,000 ohms or more have little detrimental affect on the operation of the spark plug for igniting a fuel-air mixture while providing a considerable reduction in radio frequency interference. However, where the spark plug is energized from the newer capacitive discharge ignition systems, high resistance ignition noise suppressors adversely affect the operation of many systems. Many capacitive discharge ignition systems cannot tolerate a high resistance in the high voltage output circuit because a high resistance slows the fast rise time of the ignition pulse applied to the spark gap. In addition, although capacitive discharge ignition systems are generally designed to provide higher output voltages, some systems are somewhat marginal on current available to the spark gap. This is particularly true for capacitive discharge ignition systems commonly used on two stroke engines such as those used in outboard motors and snowmobiles. Such systems are designed with a relatively low output impedance, e.g., 50,000 ohms, to provide a fast rise time. If a high resistance ignition noise suppressor is placed in series in the high voltage circuit, the available current at the spark gap is further limited.
It has been suggested, for example, in U.S. Pat. No. 3,882,341 which issued May 6, 1975, that a low resistance inductor is effective to suppress radio frequency radiation from a high voltage ignition circuit connected to a capacitive discharge ignition system without adversely affecting the operation of the circuit. This patent shows a spark plug having a center electrode assembly including a low resistance inductor formed from wire wound on a ferrite core. The inductor is connected in series between an electrode tip and a terminal by means of either one or two springs. The inductor effectively suppresses radio frequency oscillations from the high voltage circuit without appreciably affecting the D.C. current available to the spark gap.
U.S. Pat. No. 3,267,325 discloses a spark plug having an internal oscillatory circuit. The spark plug has a center electrode including at least one inductor and at least one spark gap. The diameter of the center electrode is relatively large as compared to the average outside diameter of the insulator and a portion of the insulator is coated with a layer of silver which is electrically grounded through the spark plug shell. This coating cooperates with the large center electrode assembly for greatly increasing the capacitance between the center electrode assembly and ground to complete the oscillatory circuit. The circuit is designed to oscillate in the 2 to 20 megahertz range to provide steep voltage rises so that the plug will fire despite a shunt resistance caused by heavy fouling deposits on the insulator. However, the oscillatory circuit is a radio frequency signal generator which will inherently increase radio frequency radiation from the high voltage circuit.
Auxiliary spark gaps have also been provided in the center electrode of prior art spark plugs. The function of such gaps has been to improve the operation of the spark plug when partially fouled. Since the operating voltage must be sufficient to jump both the primary and the auxiliary spark gaps at the same time, there is a sudden current flow in the spark plug and energy will not be dissipated as readily through a shunting resistance across the primary spark gap caused by fouling deposits on the insulator.